Merge branch 'update-duration-at-the-end-of-pipeline' into 'master'

Update duration at the end of pipeline

## What does this MR do?
Moves duration calculation to be done only once at the end of pipeline processing.
Currently this is done every one build.

## Why was this MR needed?
This is the simplest thing that we can do before properly implementing duration calculation: https://gitlab.com/gitlab-org/gitlab-ce/issues/23523#note_17145614

This is ~Performance improvement that significantly affects: http://performance.gitlab.net/dashboard/db/sidekiq-workers?var-worker=PipelineUpdateWorker%23perform&var-database=Production&from=now-1h&to=now

See merge request !6987

CHANGELOG.md

@@ -23,6 +23,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Add `/projects/visible` API endpoint (Ben Boeckel)
   - Fix centering of custom header logos (Ashley Dumaine)
   - Keep around commits only pipeline creation as pipeline data doesn't change over time
+  - Update duration at the end of pipeline
   - ExpireBuildArtifactsWorker query builds table without ordering enqueuing one job per build to cleanup
   - Add group level labels. (!6425)
   - Add an example for testing a phoenix application with Gitlab CI in the docs (Manthan Mallikarjun)

app/models/ci/pipeline.rb

@@ -59,9 +59,6 @@ module Ci
 
       before_transition any => [:success, :failed, :canceled] do |pipeline|
         pipeline.finished_at = Time.now
-      end
-
-      before_transition do |pipeline|
         pipeline.update_duration
       end